Multi-resonant probe circuits for nuclear magnetic resonance (NMR) usage have been employed for decades in a wide variety of double resonance experiments. The present work is particularly directed to a class of multi-resonant probe especially distinguished by selectable use for concurrent or solo resonant capability.
Concurrently resonant probe circuits are discussed by Chance, et al J. Mag. Res., v. 65, pp. 122-129 (1985); by Slichter et al, J. Mag. Res., v. 135, pp. 273-279 (1998); and by Hu et al, Rev. Sci. Instr., v. 69, pp. 477-478 (1974) and this class of device embodies a circuit having a single port for accommodation of RF signals. The circuit exhibits distinct, well separated resonant response to RF currents of corresponding frequencies. For clarity, the greater part of the present work is described in terms of an NMR probe circuit responsive to two resonances.
Concurrently resonant NMR probes were available from both Varian Associates, Inc. and Nalorac Corp., circa 1994. Such prior art probes were often referenced as “4 nucleus probes” because two coaxial RF coils of such probes were each supported by a concurrently resonant circuit of the above type and a pair of resonant nuclei could be simultaneously manipulated/observed from one of these coils and its supporting circuit.
In both of the above prior art examples, concurrent sensitivity was achieved in a single RF port communicating with a first of a pair of coupled sub-circuits. The first (“principal”) sub-circuit includes the RF port and the NMR coil that couples to the sample, and is capable of tuning to resonant response to either of the specified resonances. The second (“auxiliary”) sub-circuit is responsive to at least one of the specified resonant frequencies. The auxiliary sub-circuit, or resonator, is electrically coupled to the primary sub-circuit, but has no coupling to the sample. Solo sensitivity was achieved in prior art by re-adjustment of the coupling between the auxiliary resonance sub-circuit until the first sub-circuit responds only to the desired (solo) resonance. The Varian circuit featured a conventional tank circuit tuned for 31P and capacitively coupled to a quarter wave stub as a second resonator to provide for 13C sensitivity. The quarter wave stub was physically removed from the circuit to obtain solo 31p-15N sensitivity. The Nalorac example was realized in a similar single RF port, two sub-circuit architecture and solo mode operation for the nuclei of gyromagnetic ratio (gamma) lower than 31p was obtained by removing the resonant structure of the auxiliary sub-circuit and re-tuning the other sub-circuit to the desired resonance. In both of these prior art examples, the coupling link between the sub-circuits was undisturbed for modal transitions in operations involving low gamma nuclei and the resonant sub-circuits were modified to attain the desired function.
Another prior art example includes an arrangement of a tuned LC circuit coupled to a ¼ wave stub wherein the ¼ wave coaxial element is constructed to for an alternative role of support for additional capacitors inserted into the LC circuit for single tuned operation, or as a switch through manipulation of the center conductor component. See U.S. Pat. No. 5,982,179, commonly assigned. In solo mode operation, the structure of the circuit/sub-circuits was drastically re-arranged.
For operations involving high gamma nuclei (1H, 19F, 3H) with modern NMR magnets, the prior art circuit considerations limit the coupling capacitance to (approximately) several tenths of picofarads compared to tens of picofarads for the low gamma nuclear resonances. Switching into solo mode operation for high gamma nuclei was not a feature of the prior art because the coupling link severely impacted the performance of the circuit.
Nuclei such as 1H and 19F are characterized by a high value of gyro-magnetic ratio (“gamma”) compared to 31p and 13C (and others) with the result that, in a given magnetic field, the high gamma nuclei resonate at much higher frequencies and present significantly different problems of RF circuit design. The concurrent 31P and 13C function was supplemented by the ability to operate alternatively in a singly resonant, or solo, mode for either of these nuclei by alteration of the resonant circuit. However, that feature was unavailable for the high frequency channel, e.g., the high gamma nuclei.
In simple summary, series coupled resonant circuits deriving RF power from a single port driving the first of these circuits demands at least four adjustments: (a and b) tuning the frequency of each resonant circuit; (c) impedance matching the port to the RF power source; and (d) adjusting the coupling. It is possible to accommodate the auxiliary sub-circuit to fixed specifications. However, the ability to retain the observable sensitivity of the first sub-circuit is lacking in the prior art because the coupling was undisturbed and the auxiliary resonator was not sufficiently isolated from the first sub-circuit
As above noted, the prior art transition between concurrent and solo mode function for low gamma nuclei necessitated altering the auxiliary sub-circuit as well as tuning of the first sub-circuit. It is most desired to retain sensitivity, to enhance the signal-to-noise performance, to minimally disturb the sub-circuit structure or frequency response and the like in transitions between concurrent and solo mode of operation for NMR experiments generally.